OXYCKX AM) ITS SALINK COM HINATH >NS 17'.) 



inconsiderably. This may either proceed from the fact that ih- 

 reaction of the substance (for example, tin, mercury, lead at a high 

 temperature, or a mixture of pyrogallol with caustic potash at the 

 ordinary temperature) evolves but little heat, or that the hoat 

 evolved is transmitted to good conductors of heat, like metals, or that 

 the combination with oxygen takes place so slowly that the heat 

 evolved succeeds in passing to the surrounding objects. Combustion 

 is only a particular, intense, and evident case of combination with 

 oxygen. Respiration is also an act of combination with oxygen ; 

 it also serves, like combustion, for the development of heat by 

 those chemical processes which are its consequences (the trans- 

 formation of oxygen into carbonic anhydride). Lavoisier enun- 

 ciated this in the clear expression, ' respiration is slow combus- 

 tion.' 



Reactions of slow combination of substances with oxygen are 

 termed oxidations. Combination of this kind (and also combustion) 

 often results in the formation of acid substances, and hence the 

 name oxygen (Sauerstoff). Combustion is only rapid oxidation. 

 Phosphorus, iron, and wine may be taken as examples of substances 

 which slowly oxidise in air at the ordinary temperature. If such a 

 substance be left in contact with a definite volume of air or oxygen, it 

 little by little absorbs the oxygen, as may be seen by the decrease in 

 volume of the gas. This slow oxidation is, as a rule, rarely accom- 

 panied by a sensible evolution of heat ; but an evolution of heat really 

 occurs, only it is not apparent to our senses, owing to the inconsider- 

 able rise of temperature which takes place ; this is owing to the 

 slow rate of the reaction and to the transmission of the heat formed as 

 radiant heat, <fcc. Thus, in the oxidation of wine and its transformation 

 into vinegar by the usual method of its preparation, the heat evolved 

 cannot be observed because it extends over whole weeks, but in the 

 so-called rapid process of the manufacture of vinegar, when a large 

 quantity of wine is comparatively rapidly oxidised, the evolution of 

 heat is quite apparent. 



Such slow processes of oxidation are always taking place in nature 

 by the action of the atmosphere. Dead organisms and the substances 

 obtained from them such as bodies of animals, wood, wool, grass, &c. 



temperature. If they vary (as Berthelot and Vieille affirm), the portion of a substance 

 which remains unburnt on explosion cannot be calculated from the pressure, and there- 

 fore the quantitative side of the subject should be considered as doubtful. But the quali- 

 tative side of the subject cannot be subject to doubt, because the dissociation of the 

 products of combustion at high temperatures is proved clearly by the most varied 

 experiments. 



x _' 



